The present invention relates to a method and device for regenerating a nickel metal hydride battery that restores the capacity of a negative electrode.
A nickel metal hydride battery includes a positive electrode of which the main component is nickel hydroxide, a negative electrode of which the main component is a hydrogen absorbing alloy, and an alkaline electrolytic solution. In the nickel metal hydride battery, the negative electrode generally has a larger capacity than the positive electrode. Thus, the discharge capacity of the battery is limited by the capacity of the positive electrode (hereinafter referred to as positive electrode restriction). In a normal nickel metal hydride battery, the negative electrode includes a non-charged portion referred to as a charge reserve, which is chargeable but not charged when the positive electrode is fully charged, and a charged portion referred to as a discharge reserve, which is dischargeable but remains in the negative electrode when the positive electrode no longer includes a charged portion. In this manner, the positive electrode restriction limits increases in the internal pressure that results from overcharging and reactions caused by overcharging.
Hydrogen absorbed by the hydrogen absorbing alloy may permeate a battery case and leak out. The hydrogen absorbing alloy absorbs hydrogen when the battery is charged and releases hydrogen when the battery is discharged. When hydrogen permeates the battery case and leaks out, the hydrogen is released from the hydrogen absorbing alloy in order to maintain the hydrogen partial pressure in the case. This decreases the discharge reserve of the negative electrode. In particular, when the discharge reserve drastically decreases over a long usage period of the battery, the negative electrode restriction that limits the capacity of the negative electrode may decrease the capacity of the battery.
To solve this problem, Japanese Laid-Open Patent Publication No. 2014-186817 describes an example of a method for regenerating a battery that includes overcharging a nickel metal hydride battery and discharging hydrogen gas generated at a positive electrode out of a battery case through a safety valve arranged on the battery case. More specifically, when the nickel metal hydride battery is overcharged, a positive electrode reaction occurs in which the positive electrode generates oxygen and a negative electrode reaction occurs in which a hydrogen absorbing alloy absorbs hydrogen. When the oxygen generated at the positive electrode reacts with hydrogen absorbed by a hydrogen absorbing alloy, the hydrogen absorbing alloy returns to a state in which hydrogen is not absorbed in the hydrogen absorbing alloy. Thus, the discharge reserve does not increase. In the method described in Japanese Laid-Open Patent Publication No. 2014-186817, oxygen gas generated at the positive electrode is discharged from the safety valve. This limits the reaction of oxygen with the hydrogen absorbed in the hydrogen absorbing alloy and maintains a state in which hydrogen is absorbed in the hydrogen absorbing alloy. Further, Japanese Laid-Open Patent Publication No. 2014-186817 describes that the charge amount from when the safety valve opens is proportional to the hydrogen absorbing amount of the negative electrode and thus adjusts the charge amount in accordance with a target increase amount after opening the safety valve.